CD45, a transmembrane tyrosine phosphatase, plays a critical role in antigen receptor signaling in T and B cells by positively regulating Src kinases. The mechanisms by which CD45 function is regulated are not known. The regulated alternative splicing of exons 4, 5 and 6 in its extracellular domain and the tissue and cell-type specific expression of the resultant isoforms suggest an important function for this domain. No ligand for CD45 has been definitively identified. In previous studies using a chimeric EGFR/CD45 receptor, we showed that dimerization of the cytoplasmic domain of CD45 negatively regulates its function. Based on the crystal structure of the juxtamembrane and membrane proximal phosphatase domain of a related transmembrane tyrosine phosphatase, PTP alpha, we developed a model to explain the observed inhibitory effect of dimerization on CD45 function. This model proposes that a wedge-like structure, formed from sequences in the juxtamembrane domain, mediates dimerization-induced inhibition of catalytic function by blocking the catalytic site of the partner phosphatase. This model was supported by experiments with mutations introduced into the EGFR/CD45 chimera. We have now further tested this model by inactivating the putative inhibitory wedge in the mouse germline by introducing a single amino acid mutation, E613R. The resultant mice develop a lymphoproliferative and autoimmune syndrome that resembles systemic lupus erythematosus. The proposed studies will: 1) define the cell type driving this disease; 2) determine whether the disease is antigen driven; 3) attempt to identify the molecular basis for the downstream effects of the E613R mutation; and, 4) define the role of the alternatively spliced exons in dimerization and regulation of CD45 function.